Generation of Magnetic Fields for MRI with Loops Having Current Shunts
A conducting loop has thick cross section and is powered by a single voltage source capable of producing extremely high currents. Anti-parallel segments of the loop are brought in close proximity to each other and the unpaired segments in this loop are shaped to collectively form a homogenous B0 field. Voltage sources shunt current from one point of the thick loop to another such that the resulting redistribution of current within the thick loop allows it to simultaneously establish required gradient fields and/or shimming fields in addition to its Bo field.
This application is related to International Application No. PCT/U.S. 2012/050462, which was filed on Aug. 10, 2012 and which had claimed the benefit of U.S. Provisional Patent Application No. 61/574,823 filed on Aug. 10, 2011.
STATEMENT REGARDING FEDERALLY FUNDED RESEARCH AND DEVELOPMENTThis disclosure was not the subject of federally sponsored research or development.
TECHNICAL FIELDThe present disclosure pertains to the establishment of magnetic field patterns through the application of electrical currents; more particularly, the present disclosure pertains to the establishment of magnetic field patterns in the context of Magnetic Resonance Imaging (MRI) scanners and in the context of other systems such as Nuclear Magnetic Resonance Spectroscopy, Electron Paramagnetic Resonance Imaging, and Electron Paramagnetic Resonance Spectroscopy that also require establishment of precise magnetic field patterns for the elicitation of information from a subject.
BACKGROUND ARTA Magnetic Resonance Imaging (MRI) scanner and other similar devices are systems that establish magnetic fields so as to precisely manipulate the orientations of magnetic moments inherently present within a subject. This manipulation causes the magnetic moments to generate electrical signals within the scanner, and these signals are in turn used to construct detailed images of the internal composition of the subject.
The magnetic field seen within an MRI scanner during imaging is normally the sum of two or more very different magnetic field patterns produced by the scanner. These patterns must be carefully designed and timed so that their net effect produces the magnetic moment orientations desired at a particular instant of time within the volume of the scanner specifically designated for imaging. The magnetic field patterns considered critical to MR image acquisition are the Bo field, which is very strong and homogenous; the B1 field, which fluctuates at a radio frequency; and the x-gradient field, y-gradient field, and z-gradient field, the magnitude of each of which changes approximately linearly in the x-, y-, and z-directions, respectively. Shimming magnetic fields are very often also used, for improvement of the homogeneity of the B0 field.
Each of the above magnetic field patterns is normally produced by a distinct structure within the scanner, and each such structure is either a configuration of electrical currents or a configuration of permanent magnets. In the case of resistive MRI scanners, all of the magnetic field patterns are produced by non-superconducting electrical structures.
MRI imaging has been applied with great success to disease diagnosis. However, the extension of MRI to disease screening, including cancer screening, has unfortunately been relatively limited. Two factors significantly limiting the use of MRI for screening are the relatively high cost generally associated with scanner construction and the discomfort associated with the typically small patient space found within MRI scanners.
One approach to making scanners more inexpensive and spacious, and to therefore develop a scanner specifically oriented towards disease screening, would be to simultaneously generate a plurality of the magnetic field patterns used in MRI with a configuration carrying the sum of their respective electrical currents. It is in principle conceivable to sum the currents of the B0 field, gradient fields, and shimming fields because the vectors of all of these fields happen to be principally oriented in a single direction, by convention the z-direction.
However, while methods have been developed that appear to be very successful in producing a plurality of gradient fields and/or shimming fields with a summed current configuration, no practical means has yet been introduced to specifically combine the B0 field with gradient fields and/or shimming fields. For example, U.S. Pat. No. 6,492,817 to Gebhardt et al. shows an electrical configuration that can simultaneously establish different magnetic field patterns and that consists of a series of parallel concentric loops connected by regularly spaced line segments oriented perpendicularly to the plane of the loops. Because the currents required for a B0 field are on the order of tens of thousands of Amps when loop winding is not used, each loop in this structure contributing to a hypothetical B0 field would have to have a voltage source capable of delivering extremely large currents. Assuming a minimum of four loops for a sufficiently homogenous B0 field, four voltage sources for extremely large currents would therefore be needed for that structure to produce a B0 field among its other magnetic field patterns.
U.S. Pat. No. 6,933,724 to Watkins et al. has disclosed an electrical configuration within which individual loops have been replaced with separate loop segments or arcs with independent voltage sources. The current pattern in each segmented loop, and in the structure as a whole, is clearly able to represent a sum of current patterns associated with different MRI magnetic field patterns. However, here every segment used to contribute to a hypothetical B0 field would require a voltage source able to generate tens of thousands of Amps. Again assuming the assembly of a minimum of four loops for a B0 field, and further assuming that each segmented loop of the Watkins et al. structure consists of at least four segments, sixteen sources of extremely high current would be needed if this structure simultaneously produced a Bo field along with its other magnetic field patterns. Beyond that very impractical requirement, the extremely high return current associated with each segment contributing to a B0 field would lead to a wasting of energy and would additionally be likely to significantly distort the magnetic field within the imaging volume of the scanner.
DISCLOSUREIt is an object of this disclosure to provide a structure able to establish a B0 field along with one or more other magnetic field patterns via a summed current configuration, without requiring a plurality of voltage sources of extremely high current or having to deal with the other problems mentioned above.
This object is achieved in accordance with the present disclosure through an embodiment involving a conducting loop with thick cross section and a single voltage source capable of producing extremely high currents. Antiparallel segments of the loop are brought in close proximity to each other, meaning that the loop is effectively “pinched” at one or more locations, and each pair of antiparallel segments contributes approximately zero magnetic field within the imaging volume of the scanner. The unpaired segments in this loop are shaped to collectively form a homogenous B0 field. Voltage sources then shunt current from one point of the thick loop to another such that the resulting redistribution of current within the thick loop causes it to simultaneously establish required gradient fields and/or shimming fields in addition to its B0 field.
A still better understanding of the disclosed system and method for the simultaneous establishment of a B0 field and other magnetic field patterns with a single thick loop may be had by reference to the drawing figures wherein:
VA=(2β)RA+2(Ipolarizing+δ+β)Rq
VB=(2γ)RB+2(Ipolarizing+δ+γ)Rq
VC=(δ−β−γ)RC+4(Ipolarizing+δ)Rq,
where Rq is the resistance of each quarter of the circular structure, RA is the total resistance associated with Shunt A, RB is the total resistance associated with Shunt B, and RC is the total resistance associated with Shunt C.
Several practical notes regarding
As is well known to those skilled in the art, a structure exposed to a very strong magnetic field and also containing a current that is changing over time will generally vibrate from Lorentz forces and thereby produce acoustic noise. Segments of the thick loop 100 with changing currents would generally be expected to be immune from Lorentz forces associated with the field emanating from other segments of the thick loop simply because the thick loop will likely weigh on the order of a few thousand kilograms. The thin loop 100′ placed near a B0 field-producing structure would, on the other hand, clearly be vulnerable to Lorentz forces. One way to mitigate that problem is shown in
Those skilled in the art will understand that there are many other variations associated with the present disclosure beyond those presented in the above figures. In some embodiments, the thick loop could be made to branch and rejoin, or a plurality of thick loops could be placed together, but the overall structure of currents may still be equivalent to that described for the embodiment of
Having now disclosed the system and method of the present disclosure, those of ordinary skill in the art will understand that some or all of the advantages described in the following paragraphs may be enabled. In the following paragraphs, the physical embodiment of the circuit drawn in
A first advantage of a thick-loop scanner can be seen from the fact that, given that the precision of the B0 field magnetic field pattern is particularly important in MRI, the loops of a thick-loop scanner will likely be designed to have positions, diameters, and thicknesses equal or approximately equal to the positions, diameters, and thicknesses of a typical B0 field-producing structure in a resistive MRI scanner. This means that, assuming that the paths of the shunts are set to be outside of the volume enclosed by the thick loops as in
A second advantageous feature of a thick-loop scanner is the relatively low manufacturing cost expected. Only one significant magnetic field-producing structure other than the B1 field-producing structure would have to be manufactured for the scanner. Furthermore, the thick loop would presumably be assembled from molded pieces and consequently be more cost-effective to make in comparison to structures formed from the careful, repeated winding of wires. Molded structures may also be less susceptible to errors arising from the mechanical stresses of transport than wound structures are, and for that reason it might be more economical to disassemble a thick-loop scanner and reassemble it elsewhere, for example, for donation to a developing nation, than would be the case for a scanner with a large number of windings. It is true that current shunts will have to be manufactured along with the thick loop of a thick-loop scanner, and attached to that thick loop; however, like the thick loop itself, the current shunts are relatively simple structures.
A third advantageous feature of a thick-loop scanner is its capability to provide relatively quiet operation. In standard MRI, the different structures are often placed within one another in the form of tightly-fitting concentric cylinders; however, as, explained above, the thick-loop scanner will be expected to have a relatively large amount of free space. Part of this increased space could be devoted to the placement of slender evacuated tubes around the current shunts, which would significantly reduce the noise transmission resulting from Lorentz forces acting on the shunts when their currents change in value. If the shunts happen to have the arrangement depicted in
Having now read and understood the disclosed system and method for the simultaneous establishment of a B0 field and other magnetic field patterns, those of ordinary skill in the art will recognize other advantages, variations, and embodiments that have been enabled by the foregoing disclosure. Such advantages, variations, and embodiments shall be considered to be part of the scope and meaning of the appended claims and their legal equivalents.
Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure.
The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Various advantages of the present disclosure have been described herein, but embodiments covered by the claims may provide some, all, or none of such advantages.
Claims
1. A magnetic resonance imaging (MRI) device, comprising:
- a conductor operable to carry a current between first and second terminals of a loop voltage source, wherein the conductor comprises:
- a plurality of loop portions disposed about a central axis, wherein the plurality of loop portions includes a first loop portion coupled to the first terminal of the loop voltage source and a second loop portion coupled to the second terminal of the loop voltage source by a return portion of the conductor;
- one or more inter-loop portions connecting the plurality of loop portions in series;
- a first plurality of current shunts coupled between first and second nodes of respective ones of the plurality of loop portions, wherein each of the first plurality of current shunts includes a single voltage source;
- a second plurality of current shunts coupled between third and fourth nodes of respective ones of the plurality of loop portions, wherein each of the second plurality of current shunts includes a single voltage source;
- a third plurality of current shunts coupled between two nodes respectively on two interloop portions connected to ones of the plurality of loop portions, wherein each of the third plurality of current shunts includes a single voltage source;
- wherein a magnetic field of the MRI device is generated substantially by the plurality of loop portions of the conductor.
2. The apparatus of claim 1, wherein the conductor carries a polarizing magnetic field current of at least 10,000 amps.
3. The apparatus of claim 1, wherein the selected respective currents are operable to create at least one gradient magnetic field.
4. The apparatus of claim 1, wherein the selected respective currents are operable to create at least two gradient magnetic fields.
5. The apparatus of claim 1, wherein the selected respective currents are operable to create at least three gradient magnetic fields.
6. The apparatus of claim 1, wherein the selected respective currents are operable to create at least one shimming magnetic field.
7. The apparatus of claim 1, wherein any net contaminating magnetic field arising from the inter-loop and return portions of the conductor has a magnitude of less than 1 part per million with respect to the polarizing magnetic field within the imaging volume of the scanner.
8. The apparatus of claim 1, wherein any net contaminating magnetic field arising from the inter-loop and return portions of the conductor has a magnitude of less than 5 parts per million with respect to the polarizing magnetic field within the imaging volume of the scanner.
9. The apparatus of claim 1, wherein any net contaminating magnetic field arising from the inter-loop and return portions of the conductor has a magnitude of less than 50 parts per million with respect to the polarizing magnetic field within the imaging volume of the scanner.
10. The apparatus of claim 1, wherein any net contaminating magnetic field arising from the inter-loop and return portions of the conductor has a magnitude of less than 100 parts per million with respect to the polarizing magnetic field within the imaging volume of the scanner.
Type: Application
Filed: Feb 14, 2013
Publication Date: Dec 31, 2015
Inventor: Hardave S. KHARBANDA (San Antonio, TX)
Application Number: 14/768,215